'The Walking Dead' finally reveals one of the biggest mysteries fans have been wondering since the show began

Sunday's episode of "The Walking Dead" saw the return of Negan reuniting with Rick and his group for the first time since the season seven premiere. While their reunion was anything but rosy, fans were in for a shock when the show ended up delivering an unexpected reveal that had nothing to do with Negan, the Saviors, or any other future characters.

Near the end of the extended 90-minute episode, Rick revealed the answer to one of the biggest mysteries the show has had since its first season: Who is the true father of his daughter Judith?

Last chance to head back before spoilers.

Old school "The Walking Dead" photo from season two with Rick's old pal Shane and his wife Lori.
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While telling Michonne that they need to accept the way the world is right now, with Negan in it, Rick told Michonne that he previously had to accept that the daughter who he has been raising isn't his.

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Yes, Rick has known all along that his daughter with Lori actually belonged to his best friend Shane.

Here's the speech Rick gave to Michonne during the big reveal:

I had a friend. I don't talk about him. He was my partner.

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He got Lori and Carl to safety right after it all started. I couldn't. I was in the hospital. I didn't know what was happening.

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My friend, his name was Shane, him and Lori they were together. They thought I was dead.

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I know Judith isn't mine. I know it. I love her. She's my daughter, but she isn't mine. I had to accept that. I did. So I could keep her alive. I'll die before she does and I hope that's a long time from now, so I can raise her and protect her and teach her how to survive. This is how we live now. I had to accept that too so I could keep everyone else alive.

Both of Rick's children, Carl and Judith.
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Rick's explanation to Michonne echoes what Lori said to Shane after he found out she was pregnant: that their child would always be Rick's.

"Even if it's yours, it's not gonna be yours," Lori told Shane in season two, episode seven. "It's never gonna be yours and there's nothing you can do to change that."

"It's one of those watershed moments that happens, certainly between the two lovers, between Michonne and Rick, that brings them together," said Lincoln. "It's a very important moment in understanding his psyche and going, 'This is what I've been doing for the last two years. I've accepted the situation and I will accept the situation if it's going to save lives and if it's going to protect the people I love. I will take it.' It's another perfect kind of quiet heroism. You know, he's a stepdad and it doesn't stop him loving his child, but he kind of has accepted that it's not his."

"The choice he [Rick] made with the Saviors, it was a very, very interesting and brilliant way of echoing that heroism that he'd shown with his own child — that he had sucked it up and he just said, it doesn't matter. It doesn't matter. I love her. She's my daughter. Even if she's not mine, she's still my daughter," Lincoln said in a taped segment on Sunday's "Walking Dead" aftershow, "Talking Dead." "I thought that was a brilliant way of explaining what Rick was enduring and capable of enduring."

Executive producer David Alpert said for Rick, the decision to reveal Judith's paternity to Michonne is about showing her what the group's priorities are at the moment. Instead of being reckless and seeking vengeance right away, they need to be mindful of what's actually important.

"[Rick] comes out of the coma, he finds his wife, he finds his kid, he finds his friend and he comes to that place and he realizes, you know what, everything else has gone away and I've found them and that's what's important," said Alpert. "Whether or not this is or is not my biological daughter is not important. This is the thing that's important. What he's trying to communicate to Michonne is, 'Listen. We have to survive another day. If we don't survive another day, then nothing matters.' That's what he's trying to get out there."

Alpert added that he didn't think there would ever be a reason for Rick to ever tell Judith that he's not her father.

"Unless she grows up and she has that big shane beard," he joked.

While fans weren't expecting the reveal in Sunday's episode, the news that Rick isn't Judith's father probably didn't surprise too many. It's been a topic of discussion for years within the fandom.

In 2012, TV Guide asked comic creator Robert Kirkman if we may ever learn the identity of Judith's real father and he hinted that Shane was indeed the dad.

"I'd prefer to keep that kind of thing somewhat ambiguous, but I will say if you've watched this show, it's definitely much more likely that it is Shane's baby, but you never really know," Kirkman said at the time.

Though his character has been dead on "The Walking Dead" for years, even actor Jon Bernthal, who played Shane, guessed that Judith was his daughter several times.

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The actor told fans Judith was "100% Shane's" at Wizard World Chicago Comic Con in 2013. During an appearance at October's New York Comic Con, the actor told fans the same when he was asked Judith's paternity.

Lincoln told EW he actually made the big reveal to Bernthal awhile back so he probably knew when he was joking around with fans at Comic Con.

"'Dude, you're never going to believe this,' and he was like, 'Oh, my God, no. What?' said Lincoln. "It was hilarious because he was like, 'No. What… you… really? Really?' And he took it all so perfect. It was so Jon. It was beautiful because it was almost like he wanted to replay his scene again. He wanted to go back and reshoot the thing and I was like, 'Dude, I know. Come on… they… you know.'"

As fans have pointed out, it logically made more sense that Judith was Shane's child. It's estimated that Rick was in his coma for four to five weeks and then a few more weeks until he was finally reunited with Lori, his son Carl, and Shane. By that point, fans believe Shane was with Lori for the better part of two months.

When the question was asked on Reddit over a year ago, user TsarNab came up with a pretty convincing timeline in which it was determined that Rick returns to his family on day 60 of the outbreak. It's on that night he has sex with his wife. Eight days later, Lori takes a pregnancy test that's revealed positive. It's determined that eight days later would probably be too soon for an over-the-counter pregnancy test to reveal positive results and so it was largely assumed by fans that Lori probably conceived a while ago and that Rick wasn't the father. You can read TsarNab's full breakdown here.